Liquid crystal displays (LCD) or organic light-emitting diode (OLED) displays have advantages such as a low radiation, a small volume, a low power consumption and the like, and thus replace conventional cathode ray tube (CRT) display gradually and are widely applied to information products such as notebook computers, personal digital assistants (PDA), flat-screen televisions, mobile phones or the like. In a conventional liquid crystal display, driving chips outside a liquid crystal panel are utilized to drive chips on the liquid crystal panel to display an image. In order to reduce the number of components and decrease the manufacture cost, in recent years, it is a trend that the driving circuit is manufactured on the liquid crystal panel directly; for example, the technique that integrates the gate driver into the liquid crystal panel (gate on array, GOA) is adopted.
A commonly-used shift register unit shown in FIG. 1 comprises a transistor T1, a transistor T2, a transistor T3, a capacitor C1 and a pull-down circuit 11. Assuming that the shift register unit shown in FIG. 1 is a Nth stage of shift register unit, a gate of the transistor T1 receives an output signal of a (N−1)th stage of shift register unit, a source of the transistor T1 receives a signal VDD, and a drain of the transistor T1 is connected to a gate of the transistor T2; a source of the transistor T2 receives a clock signal CLK, and a drain of the transistor T2 serves as an output terminal of the Nth stage of shift register unit; one terminal of the capacitor C1 is connected to the gate of the transistor T2, and the other terminal of the capacitor C1 is connected to the drain of the transistor T2; a source of the transistor T3 is connected to the gate of the transistor T2, a gate of the transistor T3 receives an output signal of the (N+1)th stage of shift register unit, and a drain of the transistor T3 receives a signal VSS; the pull-down circuit is connected to the gate of the transistor T2 and the drain of the transistor T2.
When the transistor T1 is turned on by the output signal from the (N−1)th stage of shift register unit, the gate of the transistor T2 receives the signal VDD, the transistor T2 is turned on, and the Nth stage of shift register unit outputs the signal CLK. At this time, the (N+1)th stage of shift register unit receives the signal CLK outputted from the Nth stage of shift register unit and also outputs the signal CLK, so that the transistor T3 is turned on, and the gate of the transistor T2 receives the signal VSS, and the transistor T2 is turned off. After that, the pull-down circuit 11 is configured to allow the Nth stage of shift register unit to output a voltage stably.
A display device adopting such shift register units only scans in a specific direction, for example, scans a first gate line to an Nth gate line in sequence. In some special cases, however, it needs to turn over the picture displayed on the display device by 180°, the display device adopting such shift register units can not realize such a function.